Every fragment feels a real inverse-square gravity from the central body:
a = -GM · r̂ / |r|²
which is integrated every frame (sub-stepped for stability), so fragments trace genuine Kepler-like orbits instead of a scripted path — raise the launch speed of a fragment above the local circular speed v = √(GM/r) and it stretches into a longer ellipse; drop it below and the ellipse tightens.
Inside a thin atmosphere shell near the planet, drag applies a force opposing velocity, scaled by an exponential density falloff ρ = e^(-(r-R)/H) — this bleeds orbital energy from low fragments so they spiral in and re-enter, mirroring how real LEO debris eventually decays.
When Cascade is on, two fragments that collide don't just vanish — they shatter into several new fragments with perturbed velocities, which can themselves collide again: a simplified model of the real Kessler syndrome, where debris density growing dense enough triggers a self-sustaining collision cascade.